This invention relates in general to the combustion of fluent fuels and more particularly to a control device for proportioning the amount of fuel to the amount of combustion air over a wide range of heat delivered.
Many large buildings and certain industrial processes utilize steam, which is produced in boilers, as a medium for transferring heat to remote locations. Within the typical boiler a combustible fuel, which may be a gas or a liquid, and air mix, and the mixture burns to produce the heat required for converting water to steam. The fuel flows into a combustion chamber within the boiler through a fuel line, whereas the air is forced into the combustion chamber by a fan. Because the demands on such boilers often vary, it is customary to have a control valve in the fuel line and a damper immediately beyond the fan, and to operate the two in unison with a single modulating motor. Thus, when the boiler receives a demand for more steam, the modulating motor opens the fuel valve and the air damper, so that the combustion within the boiler produces more heat--and the reverse holds true as well.
The efficiency with which a boiler operates at any setting depends to a large measure on the ratio of fuel to air within its combustion chamber. For example, it is desirable to have minimum excess of oxygen to prevent the formation of carbon monoxide and soot and yet to maximize efficiency. The more closely controlled the fuel to air mixture, the more efficient an excessive supply of oxygen causes much of the heat to be wasted merely elevating the temperature of the excess air.
Control devices exist which closely match the flow of fuel to the flow of air over a wide range of heat demands on a boiler, and these devices, to a measure, are adjustable so that at a particular damper setting for air flow, the fuel flow may be increased or decreased to achieve that proper air-fuel ratio at that setting. Typically, this type of control device has a cam along which a follower rides, the follower in turn being coupled to the fuel valve to control the amount of fuel admitted to the combustion chamber. The shape of the cam may be altered to achieve the precise control at the various setting throughout the heat range, and in several devices this alteration is effected by a series of set screws located along the cam. For example, in one control device, the cam surface, over which a follower rides, lies along a coil spring that is stretched over the ends of set screws, and indeed the ends of the screws are contoured to accommodate the cylindrical shape of the spring. By turning the screws, the shape of the spring--and likewise the profile of the cam--changes, but by reason of the contours at their ends, the screws must be turned in 180.degree. increments. This detracts from the precision with which the adjustments are made. Furthermore, the spring only produces a convex profile throughout its length. It cannot provide concave segment in its profile, and where a concave segment is required for a particular burner setting, the burner must operate at less than optimum efficiency at that setting. Also, the spring is not very sturdy and must be replaced from time to time. Another control device utilizes a warp plate for its cam, with curvature of the plate being controlled by set screws. The plate does not easily accept changes in its contour and, furthermore, is subject to fracture.
The typical control device of current manufacture is designed for use on one type of furnace, generally as original equipment. Only with substantial and time consuming modifications--and considerable innovation as well--can it be applied to another furnace. Yet the fuel valves used with all furnaces are essentially the same.
The present invention resides in a control device for accurately proportioning the flow of fuel to the flow of air supplied to a combustion chamber so that the combustion within the chamber occurs under optimum conditions over a wide range of heat settings. The control device provides a camming surface over which a follower rides, and the follower operates a fuel valve. The contour of the camming surface is easily and precisely adjusted so that the proper amount of fuel enters the combustion chamber for any and all heat settings, and the adjustment permits a concave segment on the surface if necessary. The device mounts on the fuel line itself, next to the fuel valve, and may be fitted to a wide variety of heating devices with little effort. It is simple in design and durable in construction.